Thermoplastic resins such as polyethylene terephthalate (PET) are commonly used to make a variety of different types of packaging materials and storage containers. PET produces high strength packaging articles and has found widespread use in applications for bottling substances such as soft drinks and water. However, because PET polymers are porous to gases such as oxygen, this has limited their use in bottling beer, fruit, and other substances susceptible to degradation by oxygen. As a result, glass and metal containers continue to dominate the market for bottling of beer and juice.
To address this limitation and improve shelf life for oxygen sensitive products (e.g., certain foods, beverages and medicines), a number of strategies have been used. One of these is the use of a physical barrier. PET containers may contain multi-layer walls or one or more oxygen scavengers to prevent oxygen from reaching the contents of the container. In some instances, a passive oxygen barrier layer has been used in a polymer container to block oxygen transmission through the container wall. For example, in a multi-layer bottle, a barrier layer made from a substance that functions as a gas barrier such as ethylene vinyl alcohol (EVA), polyvinylidene dichloride (PVDC), or Nylon MXD6 may combined with one or more layers of PET. In addition to the added complexity, multi-layer constructions may lead to delamination, or increased cost, and do not fully address the problem as these may allow oxygen already present in the container material to reach contents of the container.
Another strategy is the use of an active oxygen scavenger to reduce or deplete oxygen in the environment of the oxygen-sensitive substance (whether through the environment or from the polymeric container itself). In some cases, an oxygen scavenger may be placed within a packet which is placed within the container so as to take up oxygen. However, these packets are generally limited to solid substances and solid foods as care must be taken so that the packet is not mistakenly used or ingested. In some instances, an active oxygen scavenger is incorporated into a polymer resin that forms one or more walls of a container. Examples include inorganic materials such as reduced metal powders or certain polymers. Reduced iron powder is commonly used for oxygen scavenging in food packages, where the iron reacts with oxygen and forms iron oxide. Polyamides or polyolefins may be incorporated into the backbone of a polymer forming container walls or used to make an oxygen absorbing layer in a multi-layer package wall.
A need exists for improved active oxygen scavenging additives for polymers, and active oxygen scavenging polymers. A need exists for improved organic oxygen scavengers that maintain the desired aesthetic qualities (e.g., those that can be used in clear containers without producing undesired haze or coloring). Finally, a need exists for active oxygen scavenging polymer additives and polymers that can be derived from renewable carbon sources.